Mastering E-Prime: Meaning of all time audit measures.

Thu Feb 21 22:56:52 UTC 2013

Justine,

No programming system can exactly control the actual onset time of 
stimuli, due to the nature and limitations of stimulus hardware.  For 
visual stimuli in particular, the video frame rate plays one limiting 
role -- please see the "Critical Timing" chapter in the User's Guide 
that came with E-Prime (and/or take my online video course, see link below).

If one wished merely to *know* when a visual stimulus got presented 
(as distinct from *controlling* its onset time), one might use the 
output of a high-speed photodector aimed at a choice spot on the 
display.  Technically speaking, even then the time of stimulus 
presentation to the subject depends on things such as the distance 
from the display to the subject, the speed of light, photoreceptor 
response rates, nerve conduction to visual cortex, etc.  So the real 
question becomes, just how accurately do you need to control or know 
the onset time, and can we make it "good enough"?

And here we have some hope.  If you synchronize your visual stimuli 
to start with the "vertical blank", then stimulus OnsetTime should 
correspond very closely to when the system starts writing the 
stimulus screen to the top of the display -- and starting with 
EP2.0.10.182, you can even adjust this (see 
http://www.pstnet.com/support/kb.asp?TopicID=3027 ).  Now you just 
need to synchronize your EEG signals with the OnsetTime of your 
stimulus.  As you have no doubt figured out, you will be hard pressed 
to do that using WritePort.  But OnsetSignalEnabled, etc., do just 
what you need here (please see 
"RteRunnableInputObject.OnsetSignalEnabled" and related topics in the 
E-Basic Help facility).  And with EP2.0.10.182 and later you may 
accomplish the same end with the new Task Events feature, which 
should supplant OnsetSignal..., etc.  (I show how to use 
OnsetSignal... to output EEG signals in my online video course, and 
hope to include coverage of Task Events in the future.)

Finally, if you want to verify that your EEG triggers really are 
synchronized with your stimuli, then you should use an oscilloscope 
to monitor both the stimulus onset and the EEG trigger (the Black Box 
ToolKit may may this easier).

I hope that helps.

-----
David McFarlane
E-Prime training 
online:  http://psychology.msu.edu/Workshops_Courses/eprime.aspx
Twitter:  @EPrimeMaster (https://twitter.com/EPrimeMaster)

At 2/15/2013 01:47 PM Friday, justine wrote:
>I am in the process of conducting my own timing tests (for an 
>Eprime/NetStation experiment), and this was invaluable in trying to 
>understand the time audit measures. I gather from this that there is 
>no way of knowing when the stimulus was *actually* presented on the 
>screen (given the refresh rate). I assume this means that my 
>triggers in the EEG record could be off by whatever that screen 
>refresh delay was. Is that correct?
>
>Thanks,
>
>Justine VanDyke
>Postdoctoral Research Fellow
>University of South Carolina
>
>On Friday, 10 September 2010 09:46:03 UTC-4, David McFarlane wrote:
>When you look at the Logging tab on the properties page of any
>stimulus object, you will find a host of items available for
>logging.  Most of these are time audit data.  But what do all these
>items mean, and what are they good for?  Chapter 3 of the E-Prime
>User's Guide discusses time auditing to some degree, and the timing
>diagram at Appendix E provides one way to see the relationships
>between these items.  As an alternative, here I try to set out, in
>order, a brief description of these items.
>
>First let us distinguish between timing control *settings* and time
>audit *measures*.  The following items do not reflect any results
>formed during the course of a stimulus but simply log the settings
>provided by the user (e.g., you).  You may choose to have any of
>these logged just to keep a record of settings active during the experiment:
>- Duration:  To reiterate, this does *not* show the actual duration of
>    the stimulus, only the setting as provided by the user.
>- PreRelease:  Affects the TargetOffsetTime (see below).
>- TimingMode:   Event, Cumulative, or Custom, as set by the user (see
>    the online E-Basic Help).
>- CustomOffsetTime:  In Custom timing mode, overrides the
>    TargetOnsetTime (see the online E-Basic Help).
>- CustomOnsetTime:  In Custom timing mode, overrides the
>    TargetOffsetTime (see the online E-Basic Help).
>
>Now, the raw time audit measures, listed in the order in which events
>occur during the execution of a stimulus object.  These are all time
>stamps in milliseconds from the start of the current program run:
>- StartTime:  Time at which E-Prime started executing the stimulus
>    object.
>- TargetOnsetTime:  Scheduled time at which presentation of stimulus was
>    to begin; set automatically from GetNextTargetOnsetTime (see online
>    E-Basic Help).
>- OnsetTime:  Time when E-Prime actually submitted the stimulus data for
>    presentation (e.g., proceeded to copy data to display memory or load
>    sound buffer).  This may not coincide with when the stimulus actually
>    got presented, e.g., if data are submitted in the middle of a display
>    refresh cycle then they may not get presented until the next refresh.
>- ActionTime:  According to the online E-Basic Help, time at which
>    E-Prime completed the "critical action" of the stimulus.  The
>    documentation remains somewhat vague about this -- perhaps "critical
>    action" means copying data to display memory, or loading a sound or
>    video buffer.  In my tests, ActionTime never lags more than 1 ms
>    behind OnsetTime, so it serves practically the same purpose as
>    OnsetTime.
>- TargetOffsetTime:  Scheduled time at which offset actions (e.g.,
>    clean-up, ClearAfter, StopAfter) of stimulus object were to begin,
>    e.g., OnsetTime + Duration - PreRelease (Event timing mode), or
>    TargetOnsetTime + Duration - PreRelease (Cumulative timing mode).
>- OffsetTime:  Time when E-Prime actually began the offset actions of
>    the object.  Actions may not take practical effect until next
>    vertical blank, or until presentation of next stimulus.
>- FinishTime:  Time when E-Prime exited from execution of the
>    stimulus object and proceeded to execute the next section of the
>    program (e.g., next stimulus object or inline code).  Note that
>    *execution* of a stimulus *object* may end before *presentation* of
>    the *stimulus* ends; this is the point of PreRelease (as well as
>    happening as a matter of course with some stimuli such as some
>    sounds).
>
>Finally, a few composite time audit measures derived from the raw
>measures above and provided for convenience:
>- OnsetDelay = OnsetTime - TargetOnsetTime
>- ActionDelay = ActionTime - OnsetTime
>- OffsetDelay = OffsetTime - TargetOffsetTime
>- DurationError = OffsetTime + PreRelease - OnsetTime - Duration
>
>Note:
>- Time audit measures include the ActionTime that follows upon
>    OnsetTime, but no corresponding item to follow upon OffsetTime.
>- No time audit item for time stamp of vertical blank, although many
>    stimuli do not take full effect until just after a vertical blank.
>
>-- David McFarlane, Professional Faultfinder

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